It was filled to the brim with water, and you can see that the water is bulging with surface tension slightly above the top of the spike. This is because the water in the tray freezes until only one point is left and, as water expands as it freezes, it gets pushed out of the hole and builds up a wall around it. As ice tends to freeze in a triangular pattern, these ice spikes tend to be triangular in cross-section.
Some time during the following hour, the water must have thawed enough to break the ice seal in the box. This caused the water level in the spike to drop. The following two pictures were taken at 10:30am.